1. Field of the Invention:
The present invention is directed to apparatus for drilling a well into earth formations lying below a body of water, wherein the wellhead equipment of the well is positioned below the surface of the water. The well is drilled from a floating drilling vessel, with a riser connecting the vessel drilling equipment to the wellhead assembly. The riser's upper elements move relative to the offshore vessel, as the vessel rises and falls on subsequent wave crests.
2. Description of the Prior Art:
An increasing amount of offshore deepwater exploratory well drilling is being conducted in an attempt to locate oil and gas reservoirs. These exploratory wells are generally drilled from floating vessels. As in any drilling operation, drilling fluid must be circulated through the drill bit in order to cool the bit and to carry away the cuttings. This drilling fluid is normally returned to the floating vessel by means of a large diameter pipe, known as a riser, which extends between the subsea wellhead assembly and the floating vessel. The lower end of this riser is connected to the wellhead assembly which is generally adjacent to the ocean floor, and the upper end usually extends through a centrally located hull opening of the floating vessel. A drillstring extends downward through the riser into earth formations lying below the body of water, and drilling fluids circulate downwardly through the drillstring, out through the drilling bit, and then upwardly through the annular space between the drillstring and riser, returning to the vessel.
As the water depths for these drilling operations continue to increase, the length of the riser and subsequently its unsupported weight also increases. Since the riser has the same structural buckling characteristics as a vertical column, riser structural failure may result if compressive stresses in the elements of the riser exceed the metallurgical limitations of the riser material. To avoid the possibility of this occurrence, riser tensioning systems are installed on board the vessel, which apply an upward force to the upper end of the riser, usually by means of cable and sheave mechanisms connected between the vessel and the upper elements of the riser.
Since the riser is secured at its lower end to the wellhead assembly, the floating vessel will move relative to the upper end of the riser due to wind, wave, and tide oscillations normally encountered in the marine environment.
This creates a problem because the stationary riser located within the hull opening of the oscillating vessel can contact and damage the vessel, unless it remains safely positioned within the hull opening. For this reason, motion compensating equipment incorporated with the riser tensioning system is used to steady the riser within the hull opening, and usually takes the form of hydraulically actuated cable and sheave mechanisms connectably engaged between the upper riser elements and the vessel structure. This equipment allows the vessel to heave, surge, and sway, without contacting the upper elements of the riser.
It is conventional practice to associate with such motion compensation equipment a riser position indicator which provides the operator with an indication of the relative positions of the offshore vessel and the riser, whereby the movements therebetween can be observed and/or recorded. In this manner, it can be determined, for example, whether or not any adjustment in the apparatus is required from time to time. In the past, the position indicators have suffered from a number of disadvantages. Not only were they relatively complicated, which in turn made them expensive and difficult to service or repair, but they were also inadequate in terms of the accuracy with which they could determine the relative position of the vessel relative to the riser. An apparatus as set forth in U.S. Pat. No. 4,421,173, issued Dec. 20, 1983, entitled "Motion Compensator with Improved Position Indicator", utilizes a flexible fluid filled tube mounted within the motion compensation apparatus structure itself. Movement of the motion compensation apparatus alters the elevated height of fluid above a differential pressure cell, thereby changing the fluid pressure sensed by the cell. A correspondingly altered signal is then generated by the differential pressure cell, as is well known to the art. Rupture of the fluid filled tube due to chafing of the tube on the compensation apparatus will cause an erroneous signal to be sent by this device. Maintenance performed on this apparatus during motion of the supporting motion compensation equipment may prove to be hazardous to operating personnel. Repair of this apparatus may therefore require suspension of drilling activities. An apparatus need be developed that provides proper position indication of the riser relative to the vessel and also allows easy repair and calibration of the device.